SOUTH PLAINFIELD, NJ - August 20, 2008 - New phase 2 data published today in The Lancet show that the investigational oral drug PTC124 demonstrates activity in nonsense-mutation cystic fibrosis (CF). The data show that treatment with PTC124 results in statistically significant improvements in the chloride channel function of patients with nonsense-mutation CF. The study was conducted at the Hadassah Hebrew University Hospital in Jerusalem, Israel and sponsored by PTC Therapeutics (PTC).

Patients with CF lack adequate levels of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, a chloride channel which is required for normal function of the lung, pancreas, liver, and other organs. Nonsense mutations are single-point alterations in the genetic code that prematurely stop the translation process, preventing production of a full-length, functional protein. Patients with nonsense-mutation CF generally make virtually no CFTR protein and thus often have a more severe form of CF. By inducing the production of functional CFTR, PTC124 is addressing the underlying genetic defect responsible for CF. Nonsense mutations are responsible for approximately 10 percent of the cases of cystic fibrosis worldwide. However, in Israel, nonsense mutations are responsible for the majority of CF cases.

This Phase 2 Israeli study enrolled 23 adult patients (median age 25 years) with nonsense-mutation CF. More than 90 percent of patients had severe CF with compromised lung function, pulmonary infection with Pseudomonas or other pathogenic bacteria, and pancreatic insufficiency. Patients were assessed in two 14-day treatment courses of oral PTC124 therapy, the first given at a lower dose and the second given at a higher dose. Results showed that at both dose levels, treatment with PTC124 was associated with statistically significant improvements (p<0.05) in CFTR-mediated chloride transport with over half of the patients entering the normal range during at least one treatment course. PTC124 induced chloride transport responses and normalization of CFTR activity across the variety of patient genotypes tested. Improvements in lung function values and body weight were also observed. PTC124 was generally well tolerated and all patients had >90 percent treatment compliance.

"This study demonstrates the potential for personalized medicine, combining selection of patients with a specific type of genetic mutation and a drug treatment that has been specifically designed to overcome that mutation," said Eitan Kerem, MD, head of pediatrics and the CF center at the Hadassah University Hospital in Mount Scopus, Jerusalem and the lead author of the study. "The publication of these ground-breaking results in the Lancet offers new hope to those patients with CF due to a nonsense mutation in the CFTR gene and establishes a path forward for evaluating the efficacy and long-term safety of PTC124."

"We are very pleased by this positive outcome from our ongoing collaboration with PTC," noted Preston Campbell, III, M.D., Executive Vice President of Medical Affairs at the Cystic Fibrosis Foundation. "The development of PTC124 fits well with our strategic goal of supporting approaches that have the potential to modify the course of CF. We are continuing to work together with PTC and the broader CF medical community to support the next steps in the evaluation of PTC124 for the clinical benefit for the treatment of nonsense-mutation CF."

"The publication of these data provides clinical proof of concept in CF for the PTC124 mechanism of action in overcoming nonsense mutations as the basis for treating genetic disease," said Stuart W. Peltz, Ph.D., President and Chief Executive Officer of PTC Therapeutics. "Based on these results, we intend to initiate a Phase 2b study later this year to evaluate the clinical benefit of PTC124 in adults and children with nonsense-mutation-mediated CF. Given the potential applicability of PTC124 to multiple genetic disorders, we have a pivotal study of PTC124 for nonsense-mutation Duchenne/Becker muscular dystrophy ongoing and are planning proof-of-concept studies in additional genetic disorders."

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The paper entitled "Effectiveness of PTC124 treatment of cystic fibrosis caused by nonsense mutations: a prospective phase II trial" is available in an advanced online publication of Lancet on Thursday, August 21st (www.lancet.com).

ABOUT CYSTIC FIBROSIS

Cystic fibrosis (CF) is a life-threatening genetic disease that causes serious lung infections and digestive complications. According to the Cystic Fibrosis Foundation, CF affects approximately 30,000 adults and children in the United States and nearly 70,000 people worldwide. There is a commercially available genetic test to determine if a patient's CF is caused by a nonsense mutation, and it is estimated that nonsense mutations are the cause of CF in approximately 10 percent of patients in the United States and Europe and over 50 percent of patients in Israel. There is currently no available therapy to correct defective CFTR production and function. Instead, available treatments for CF are designed to alleviate the symptoms of the disease. These treatments include chest physical therapy to clear the thick mucus from the lungs, antibiotics to treat lung infections, and a mucus-thinning drug designed to reduce the number of lung infections and improve lung function. In addition, the majority of cystic fibrosis patients take pancreatic enzyme supplements to assist with food absorption in digestion. There is a significant unmet medical need for treatments that address the underlying cause of CF. More information regarding CF is available through the Cystic Fibrosis Foundation (www.cff.org).

ABOUT PTC124

PTC124 is an orally delivered, investigational new drug discovered by PTC Therapeutics. The drug is being developed for the treatment of genetic disorders due to nonsense mutations. Nonsense mutations are single-point alterations in the genetic code that prematurely stop the translation process, leading to production of truncated, non-functional proteins. PTC124 induces the cellular translation machinery to read through nonsense mutations, inducing production of full-length, functional proteins. PTC124 has demonstrated proof of concept in phase 2a clinical trials. Across all clinical studies to date, PTC124 has been generally well tolerated. PTC124 is currently in phase 2b development with the goal of demonstrating that increasing functional protein levels in patients with nonsense-mediated genetic disorders will safely provide clinical benefits.

PTC124 has been granted orphan drug status by the FDA and the European Commission for the treatment of CF and DMD due to nonsense mutations. The FDA has also granted PTC124 Subpart E designation for expedited development, evaluation, and marketing.

PTC has an exclusive collaboration with Genzyme Corporation to develop and commercialize PTC124 outside the U.S. and Canada. The development of PTC124 has also been supported by grants from, the Muscular Dystrophy Association, Parent Project Muscular Dystrophy, FDA's Office of Orphan Products Development, the National Center for Research Resources and notably, the Cystic Fibrosis Foundation Therapeutics Inc. (the nonprofit affiliate of the Cystic Fibrosis Foundation), which recently expanded support of PTC124 to include funding up to $25 million.

ABOUT PTC THERAPEUTICS INC.

PTC is a biopharmaceutical company focused on the discovery, development and commercialization of orally administered, proprietary, small-molecule drugs that target post-transcriptional control processes. Post-transcriptional control processes regulate the rate and timing of protein production and are of central importance to proper cellular function. PTC's internally-discovered pipeline addresses multiple therapeutic areas, including genetic disorders, oncology and infectious diseases PTC has extensive knowledge of post-transcriptional control processes and has developed proprietary technologies that it applies in its drug discovery activities, including the Gene Expression Modulation by Small-molecules (GEMS) technology, which has been the basis for collaborations with leading biopharmaceutical companies such as Genzyme, Pfizer, Celgene, CV Therapeutics and Schering-Plough. For more information, visit the company's website www.ptcbio.com.

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